(a) Field of the Invention
The present invention relates to a norbornene based addition polymer, and more particularly to an addition polymer of norbornene based monomers containing an ester group.
(b) Description of the Related Art
Currently, polymethylmethacrylate (PMMA), polycarbonate (PC), etc. are widely used as transparent polymers. Although PMMA has good transparency, it has poor dimensional stability due to its high hygroscopicity. Therefore, it is not suitable for materials for precision optical devices or displays.
Until recently, inorganic substances such as silicon oxide or silicon nitride have been predominantly used for insulation materials. However, with the increasing need for small-sized and highly efficient devices, new highly functional materials are required.
In this regard, polymers having low dielectric constants; low hygroscopicity; superior metal adhesivity, strength, thermal stability; and transparency; and high glass transition temperatures (Tg>250° C.) attract a lot of attention. Such polymers may be used for insulation films of semiconductors or TFT-LCDs; protection films for polarizers, multichip modules, integrated circuits (ICs), and printed circuit boards; and molding compounds for electronic devices or optical materials for flat panel displays. Currently, polyimide, BCB (bis-benzocyclobutene), etc. are used as low dielectric materials for electronic devices.
Polyimide has long been used for electronic devices due to its good thermal stability and oxidative stability, high glass transition temperature, and superior mechanical properties. However, it has problems of corrosion due to high hygroscopicity, an increase in dielectric constant, its anisotropic electric property, a need for pre-treatment to reduce reaction with copper wire, its metal adhesivity, and so forth.
Although BCB has lower hygroscopicity and a lower dielectric constants than polyimide, its metal adhesivity is not good, and curing at a high temperature is required to obtain desired physical properties. Physical properties of BCB are affected by curing time and temperature.
There has been much research on cyclic olefin copolymers using transition metal catalysts. Cyclic monomers can be polymerized by ROMP (ring opening metathesis polymerization), HROMP (ring opening metathesis polymerization followed by hydrogenation), copolymerization with ethylene, and homogeneous polymerization, as shown in the following Scheme 1:

Polymers synthesized by ROMP have significantly poor thermal stability and oxidative stability due to unsaturation of the main chain, and are used as thermoplastic resins or thermosetting resins. Tenny et al. disclose, in U.S. Pat. No. 5,011,730, that a thermosetting resin prepared by the above method can be used for a circuit board, by reaction injection molding. However, as mentioned above, it has problems of thermal stability, oxidative stability, and low glass transition temperature.
There have been attempts to stabilize the main chain of the polymer by hydrogenation. Although a polymer prepared by this method has improved oxidative stability, the thermal stability is reduced. In general, hydrogenation increases the glass transition temperature of an ROMP polymer by about 50° C., but because of the ethylene groups located between the cyclic monomers, the glass transition temperature is still low (Metcon 99). Moreover, a cost increase due to increased polymerization steps and weak mechanical properties of the polymer are hindering its commercial use.
From addition copolymerization with ethylene, a product called Apel was obtained using a homogeneous vanadium catalyst. However, this method has problems of low catalytic activity and generation of excessive oligomers.
A zirconium based metallocene catalyst has been reported to give a polymer having a narrow molecular weight distribution and a large molecular weight (Plastic News, Feb. 27, 1995, p. 24). However, the activity of the catalyst decreases with an increase of cyclic monomer concentration, and the obtained copolymer has a low glass transition temperature (Tg<200° C.). In addition, although the thermal stability increases, the mechanical strength is still weak and its chemical resistance against solvents, such as halogenated hydrocarbon solvents, is poor.
Addition polymerization of norbornene using a palladium based transition metal catalyst was reported in 1967 by Union Carbide (US) [U.S. Pat. No. 3,330,815]. Although the copolymer prepared by this method has polar groups, it has a molecular weight (Mn) lower than 10,000. Later, Gaylord et al. reported polymerization of norbornene using a transition metal catalyst (Gaylord, N. G.; Deshpande, A. B.; Mandal, B. M.; Martan, M. J. Macromol. Sci.-Chem. 1977, A11(5), 1053-1070). [Pd(C6H5CN)Cl2]2 was used as the catalyst, and the yield was 33%. Later still, a norbornene polymer was prepared using a [Pd(CH3CN)4][BF4]2 catalyst, etc. (Sen, A.; Lai, T.-W. J. Am. Chem. Soc. 1981, 103, 4627-4629).
Kaminsky et al. reported homogeneous polymerization of norbornene using a zirconium based metallocene catalyst (Kaminsky, W.; Bark, A.; Drake, I. Stud. Surf. Catal. 1990, 56, 425). However, the polymer prepared by this method is very crystalline and is hardly soluble in organic solvents, and thermal decomposition occured without showing a glass transition temperature. Therefore, further studies could not be conducted.
Like the above-explained polyimide or BCB (bis-benzocyclobutene), the cyclic polymers also have poor metal adhesivity. For a polymer to be used for electronic devices, it should have good adhesivity to a variety of surfaces, such as silicon, silicon oxide, silicon nitride, alumina, copper, aluminum, gold, silver, platinum, titanium, nickel, tantalum, chromium, the polymer itself, or other polymers.
The following method was introduced to increase metal adhesivity of polyimide, BCB, etc. A substrate is treated with an organic silicon coupling agent having two functional groups, such as amino-propyltriethoxysilane or triethoxyvinylsilane, then the substrate is treated with a polymer or a polymer precursor. In this reaction, it is believed that the hydrolyzed silyl group and the hydroxy group on the substrate surface form a covalent bond.
A cyclic polymer can be used for insulating electronic devices, replacing inorganic materials such as silicon oxide or silicon nitride. For a functional polymer to be used for electronic devices, it should have a low dielectric constant; low hygroscopicity; superior metal adhesivity, strength, thermal stability, and transparency; and a high glass transition temperature (Tg>250° C.).
Such a polymer can be used for insulation films of semiconductor devices or TFT-LCDs. Here, amino groups on the substrate surface react with functional groups of the polymer or the polymer precursor to form bridges linking the substrate and the polymer. This technique has been disclosed in U.S. Pat. No. 4,831,172. However, this method is a multi-step process and requires a coupling agent.
Introduction of functional groups to a polymer comprising hydrocarbons is a useful method for the control of chemical and physical properties of the polymer. However, introduction of functional groups is not easy because unshared electron pairs of the functional groups tend to react with active catalytic sites. A polymer obtained by polymerizing cyclic monomers having functional groups has a low molecular weight (U.S. Pat. No. 3,330,815).
In order to overcome this problem, a method of adding the monomers having functional groups at a later step of polymerization has been reported (U.S. Pat. No. 5,179,171). However, the thermal stability of the polymer has not increased by this method. Additionally, physical and chemical properties and metal adhesivity did not improve significantly.
As an alternative, a method of reacting functional groups with a base polymer in the presence of a radical initiator has been introduced. However, this method involves problems in that the grafting site cannot be controlled, and only a small amount of radicals are grafted. The excessive radicals cut the polymers to decrease the molecular weight of the polymer, or they are not grafted to the base polymer but polymerize with other radicals.
When a polycyclic compound having a silyl group is used for an insulation film, it decomposes by moisture in the air. Furthermore, when the silyl group is reacted with metal, by-products such as water or ethanol are produced, which are not completely removed and increase the dielectric constant or cause corrosion of other metals.
In cyclic olefin monomers having polar groups such as an ester group, the functional groups increase intermolecular packing and improve adhesivity to metal substrates or other polymers, which allows them to be used for electronic devices. Therefore, polymerization or copolymerization of norbornene having an ester group has attracted continuous attention (McKeon et al., U.S. Pat. No. 3,330,815; Maezawa et al., Europe Patent No. 0445755A2; Risse et al., Macromolecules, 1996, Vol. 29, 2755-2763; Risse et al., Makromol. Chem. 1992, Vol. 193, 2915-2927; Sen et al., Organometallics 2001, Vol. 20, 2802-2812; Goodall et al., U.S. Pat. No. 5,705,503; Lipian et al., U.S. Pat. No. 6,455,650).
U.S. Pat. No. 3,330,815 discloses a polymer prepared from polymerization of norbornene based monomers having a polar group using a palladium based catalyst. However, the polymer obtained by this method has a molecular weight (Mn) lower than 10,000.
Europe Patent No. 0445755A2 discloses a copolymer of a norbornene derivative containing halogen, oxygen, or nitrogen, and a nickel and palladium based catalyst for preparing the same. However, examples of this patent do not present polymerization of a norbornene derivative having a polar group, and just present polymerization of norbornene itself.
Risse et al. reported that in polymerization of a norbornene derivative having a polar group such as an ester group using a palladium based catalyst, the presence of an endo form of ester-norbornene, which is a monomer, (endo/exo=80/20) reduces polymerization yield and molecular weight compared to when there is only an exo isomer (Makromol. Chem., 1992, Vol. 193, 2915-2927; Risse et al.; Macromolecules, 1996, Vol. 29, 2755-2763; Risse et al.).
In the report of Risse et al., the polar group is not directly connected to the norbornene based monomer, but rather through a methyl group. However, according to the report of Sen et al., the polymerization yield and the molecular weight reduce even more if the polar group is directly connected to the norbornene ring (Organometallics, 2001, Vol. 20, 2802-2812). To be specific, when a polar group such as an ester group is directly connected to the norbornene ring, the polymerization yield was lower than 40%, and the molecular weight (Mn) was below 10,000.
The reason why the polymerization yield and the molecular weight are low is that the unshared electron oxygen pair of the endo isomer of the ester binds with the vacant metal site, which blocks approach of other norbornene monomers to the metal, and therefore prevents further polymerization, as explained by Sen et al. (Organometallics, 2001, Vol. 20, 2802-2812). As such, when a monomer having a polar group is used for polymerization, it is preferable to use an exo-only or exo-rich monomer. Otherwise, the endo isomer causes problems.
Later, Risse and Goodall et al. disclosed a polymer prepared from a monomer having a polar group, the majority of which is an endo isomer (U.S. Pat. No. 5,705,503). However, when only the polar norbornene derivative such as ester is homopolymerized, a large amount of catalyst (catalyst/monomer=1/100) should be used.